Japanese Patent Application Publication Nos. 2007-165067, 2003-347071, and 2007-087637 respectively disclose light source apparatuses used for a projector apparatus. Such a light source apparatus used for a projector apparatus, which is disclosed in the Japanese Patent Application Publication No. 2007-165067, relates to a light source apparatus having a lamp, which is lighted with alternating current, wherein a color wheel is used for the projector apparatus (for example, refer to paragraph [0013] of the patent application publication). Similarly, the light source apparatus used for a projector apparatus, which is disclosed in the Japanese Patent Application Publication No. 2003-347071, relates to alight source apparatus having a lamp, which is lighted with alternating current.
A lamp of the light source apparatus, which is disclosed in the Japanese Patent Application Publication No. 2003-347071, is lighted, while it is vertically arranged, wherein when the vertically arranged lamp is lighted, time T1, during which voltage is applied to an upper electrode serving as a negative electrode, is longer than time T2, during which voltage is applied to a lower electrode serving as a negative electrode, so that it is possible to suppress a rise in temperature of the upper electrode (for example, refer to paragraph [0029] of the patent application publication). Similarly, the light source apparatus used for a projector apparatus, which is disclosed in the Japanese Patent Application Publication No. 2007-087637, relates to a light source apparatus having a lamp, which is lighted with alternating current, wherein it is possible to prevent generation of an unnecessary projection, by periodically inserting low frequency waveform in stationary frequency (refer to paragraph [0021] of the patent application publication).
In recent years, a projector apparatus is used for advertising media with an image called digital signage, wherein such media are required to be displayed in various directions or at various places because of the nature of advertisement. Naturally, such a light source apparatus for digital signage, is not set in a fixed projecting direction or a fixed projection place, so that a lamp is sometimes required to be horizontally placed as in the prior art to light the lamp, or sometimes the lamp is vertically placed to light the lamp. Thus, it is expected that such a lamp, which is provided in the light source apparatus for digital signage, can be lighted even if it is placed either horizontally or vertically. The light source apparatus disclosed in Japanese Patent Application Publication No. 2007-165067 is designed so that a lamp may be horizontally arranged. When such a lamp is horizontally arranged, a duty ratio of current supplied to the lamp (a ratio of a period, during which the polarity of the current is positive, to a period, during which the polarity of the current is negative, is calls a duty ratio) is in general approximately 1:1, so that electric energy, which flows from one of electrodes of the lamp to the other electrode, is controlled to be approximately equal to electric energy, which flows from the other electrode to the one electrode. In such a light source apparatus disclosed in Japanese Patent Application Publication No. 2007-165067, where the lamp is lighted when the lamp is vertically arranged, heat convection arises inside the lamp so that the temperature of the upper electrode becomes higher than that of the lower electrode. In such a kind of lamp, since the electrode is heated to high temperature (overheated) and, since the electrode is heated by the heat convention, there is a problem that the upper electrode is melted and damaged (lost).
The light source apparatus, which is disclosed in Japanese Patent Application Publication No. 2003-347071, is designed so that a lamp is vertically arranged, and a duty ratio of current supplied to the lamp is not set to 1:1. Thus, when the lamp is horizontally arranged in a state where the duty ratio of the current is not 1:1, there is a problem that one of electrodes is overheated, compared with the other electrode, so that the one of the electrodes may be damaged and lost. Moreover, in some of such light source apparatuses used for a projector apparatus, as shown in Japanese Patent Application Publication No. 2007-16506, light is emitted through a color wheel, which is divided into red (R), green (G), blue (B), and white (W) areas. However, when such a color wheel is used, if the polarity of current to be supplied to the lamp changes in a portion between two adjacent areas of the R, G, B, and W, a ripple arises so that the illumination of light from the lamp becomes high or low (bright or dark) temporarily. FIGS. 9A and 9B show current waveforms, which flow through a lamp of a light source apparatus, and switching timing of areas of a color wheel. Therefore, as shown in FIG. 9A, it is desirable to match switching timing of R/G/B/W areas of such a color wheel with polarity change timing. The light source apparatus, which is disclosed in Japanese Patent Application Publication No. 2003-347071, is designed so that a lamp is vertically arranged, wherein the duty ratio of current supplied to the lamp (a ratio of a positive current period to a negative current period) is not set to 1:1. Therefore, when such a light source apparatus is used for an apparatus, in which a color wheel is used, as shown in FIG. 9B, timing of the current polarity change and switching timing of the R, G, B, and W areas of the color wheel are not necessarily in agreement her. Therefore, due to a ripple, which occurs when the current polarity changes, the illumination of light from the lamp becomes high or low (bright or dark) temporarily, which causes a flicker.
Moreover, although an image on the entire liquid crystal display is updated at a fixed cycle (refresh rate), if change timing of the polarity of current impressed to electrodes of a lamp is not synchronized with the refresh rate (vertical frequency), a flicker may occur as in the case of the above-mentioned color wheel. As disclosed in Japanese Patent Application Publication No. 2003-347071, since the duty ratio of current is not necessarily set to 1:1 as shown in FIG. 9B, the timing, at which an image of the entire liquid crystal display changes and the current polarity change timing are not necessarily in agreement, so that the illumination of light from a lamp may become high or low (bright or dark) temporarily, which causes a flicker, as in the case of the above-mentioned color wheel.
As mentioned above, the light source apparatus of the prior art is not configured so that a lamp can be lighted in either horizontal arrangement or vertical arrangement, so that if a light source apparatus, which is designed to be horizontally placed, is vertically placed and lighted, an electrode located in an upper side may be melt and damaged. Moreover, as disclosed in Japanese Patent Application Publication No. 2003-347071, it is proposed that a lamp can be used in vertical arrangement, by setting a period, during which the polarity of current supplied to the lamp is positive to be different from a negative current period. However, when a color wheel or a liquid crystal display is used, current polarity change timing is not necessarily in agreement with the change timing of the R/G/B/W areas of the color wheel, or the refreshment timing of a liquid crystal display, so that there is a problem that a flicker occurs on the screen.
The present invention was made to solve the above mentioned problem of the prior art, and it is an object of the present invention to offer a light source apparatus capable of lighting a lamp without the problem of damages (or loss) of electrodes in either horizontal or vertical arrangement. It is another object of the present invention to offer a light source apparatus capable of displaying an image without a flicker on a screen even though the light source apparatus is applied to an apparatus using a color wheel or a liquid crystal display.